The long-term goals of this research are to develop a greater understanding of the mechanisms involved in loss and activation of primordial follicles; and to determine how to maintain healthy and viable oocytes as animals age. The ability to control loss or activation of primordial follicles gives the potential for decreasing problems associated with irregular estrous cycles. If menopause is caused in the majority of women by depletion of ovarian reserves of follicles, then decreasing rate at which follicles are lost would be beneficial in delaying menopause. A delay in menopause would also have the potential of delaying some postmenopausal problems, such as osteoporosis. In all four experiments, the concentrations of biologically active FSH in blood will be determined and correlated with numbers of primordial follicles in ovaries. The major objective of experiment 1, is to determine the effects of manipulating concentrations of FSH during the prepubertal period on loss of primordial follicles. This period of time was chosen because of the extremely rapid loss of follicles that occurs at this age. A small decrease in rate of loss of follicles would potentially affect reproductive life. Experiment 2 was designed to understand if GnRH antagonist or FSH infusion acted on primordial follicles in a random fashion, or whether treatment acted on a specific population of follicles that were selected because of the time that they initially entered meiotic arrest. Experiments 3 and 4 are designed to better understand the inheritance pattern in rate of loss of primordial follicles and the potential location of genes that have this effect.